Nowadays use of the optical communication utilizing infrared rays or a visible light is spreading. In this event, the infrared-ray communication has such problems that, since transmission is not performed at high electric power from the perspective of eye safety (eye protection), a communication speed cannot be increased, etc.
In contrast, the visible light communication uses a visible light device such as an LED (Light Emitting Diode) that can vary the color as a light source. Therefore, this visible light communication possesses such advantages that the above problem in the infrared communication can be solved, and data can be transmitted by utilizing a characteristic that the visible light device can be turned ON/OFF at a high rate, etc. (see Non-Patent Literature 1).
Such visible light device is used as not only for the optical communication but also the illumination light.
For example, in the apparatus set forth in Patent Literature 1, three types of LEDs that respectively emit lights of red (R), green (G), blue (B) color of three primary colors of light are used in such a way that thee apparatus illuminates white light by a mixed color and also performs multicolor communication by superimposing respective individual data on each of the LEDs.
In this case, a light quantity used in the visible light communication is required to satisfy satisfactorily a light quantity as the illumination light.
For example, in the apparatus set forth in Patent Literature 2, the LED is caused to emit light based on the reverse pulse position modulation in which ON/OFF positions of the pulse signal train are inverted. In this pulse signal train, the LED emits a light (ON) in a pulse range and the LED does not emit light (OFF) in a flat range. Therefore, ON time period of the pulse is prolonged to improve a luminous intensity of illumination in performing communication.
Patent Literature 1: JP-A-2002-290335
Patent Literature 2: JP-A-2004-72365
Non-Patent Literature 1: “What is Visible Light Communication”, [online], Visible Light Communication Consortium, Sep. 7, 2004 Searched, Internet <http://www.vlcc.net/about.html>